Roberto Contro

Tensile and compressive behaviour of the bovine periodontal ligament.

The mechanical response of the bovine periodontal ligament (PDL) subjected to uniaxial tension and compression is reported. Several sections normal to the longitudinal axis of bovine incisors and molars were extracted from different depths. Specimens with dimensions 10 x 5 x 2 mm including dentine, PDL and alveolar bone were obtained from these sections. Scanning electron microscopy suggested a strong similarity between the bovine PDL and the human PDL microstructure described in the literature. The prepared specimens were tested in a custom made uniaxial testing machine. They were clamped on their bone and dentine extremities and immersed in a saline solution at 37 degrees C. Stress-strain curves indicated that the PDL is characterized by a non-linear and time-dependent mechanical behaviour with the typical features of collagenous soft tissues. The curves exhibited hysteresis and preconditioning effects. The mechanical parameters evaluated in tension were maximum tangent modulus, strength, maximizer strain and strain energy density. For the molars, all these parameters increased with depth except for the apical region. For the incisors, all parameters increased with depth except ultimate strain which decreased. It was assumed that collagen fibre density and orientation were responsible for these findings.

Lumbar Dura Mater Biomechanics: Experimental Characterization and Scanning Electron Microscopy Observations

There is no consensus about the anatomical structure of human dura mater. In particular, the orientation of collagen fibers, which are responsible for biomechanical behavior, is still controversial. The aim of this work was to evaluate the mechanical properties and the microstructure of the lumbar dura mater. We performed experimental mechanical characterization in longitudinal and circumferential directions and a scanning electron microscopy observation of the tissue. Specimens of human dura mater were removed from the dorsal-lumbar region (T12-L4/L5) of six subjects at autopsy; specimens of bovine dorsal-lumbar dura mater were obtained from two animals at slaughter. Human and bovine tissues both exhibited stronger tensile strength and stiffness in the longitudinal than in the circumferential direction. Scanning electron microscopy observations of dura mater showed that the collagen fibers are mainly oriented in a longitudinal direction, which accounts for its stronger tensile strength in this direction. We conclude that dura mater has a different mechanical response in the two directions investigated because the fiber orientation is predominantly longitudinal. Implications: In this experimental work, we studied the structural and functional relationship of human lumbar dura mater. We performed mechanical tests and microscopic observations on dura mater samples. The results show that the dura mater is mainly composed of longitudinally oriented collagen fibers, which account for higher tissue resistance in this direction. (Anesth Analg 1999;88:1317‐21)

Experimental and computational approach for the evaluation of the biomechanical effects of dental bridge misfit

Dental bridges supported by osseointegrated implants are commonly used to treat the partially or completely edentulous jaw. The bridges are manufactured in metal alloy using a sequence of technological steps which well match the requirement to get custom overstructures but does not guarantee geometrical and dimensional tolerances. Dentists often experience that a perfect fit of the bridge with the abutments is almost impossible to achieve. When a misfitting bridge is forced on the abutments, deformations may occur inducing a permanent preload at the fixture-bone interface and the greater the misfit the greater is the preload and the risk of implant failure. This work gives an evaluation of the biomechanical effects induced by a misfitting bridge when forced on two supporting dental implants. The strains induced in the bridge have been measured using two purposely designed and fabricated experimental devices allowing different types of misfit. FEM 3D models of the bridge and of the bridge anchored to the bone by implants have been developed. The former has been validated by simulating the same loading conditions as in the experimental tests and comparing the bridge strains. Both models have been used for the evaluation of the stress induced in the bridge and at the fixture-bone interface by bridge length errors. The results show that the method may help to estimate the stress distribution in the bridge and bone as a consequence of different dental bridge misfits.

A finite element model of the L4–L5 spinal motion segment: biomechanical compatibility of an interspinous device

The biomechanical compatibility of an interspinous device, used for the “dynamic stabilization” of a diseased spinal motion segment, was investigated. The behaviour of an implant made of titanium based alloy (Ti6Al4V) and that of an implant made of a super-elastic alloy (Ni–Ti) have been compared. The assessment of the biomechanical compatibility was achieved by means of the finite element method, in which suitable constitutive laws have been adopted for the annulus fibrosus and for the metal alloys. The model was aimed at simulating the healthy, the nucleotomized and the treated L4–L5 lumbar segment, subjected to compressive force and flexion-extension as well as lateral flexion moments. The computational model has shown that both the implants were able to achieve their main design purpose, which is to diminish the forces acting on the apophyseal joints. Nevertheless, the Ni–Ti implant has shown a more physiological flexural stiffness with respect to the Ti6Al4V implant, which exhibited an excessive stiffness and permanent strains (plastic strains), even under physiological loads. The computational models presented in this paper seems to be a promising tool able to predict the effectiveness of a biomedical device and to select the materials to be used for the implant manufacturing, within an engineering approach to the clinical problem of the spinal diseases.

A PLATE MODEL FOR THE EVALUATION OF PULL-IN INSTABILITY OCCURRENCE IN ELECTROSTATIC MICROPUMP DIAPHRAGMS

This work deals with the mechanical response of circular microplates undergoing electrostatic actuation. A one degree-of-freedom model and Finite Element approaches are exploited in a nondimensional framework. First, a quasi-static conventional approach is adopted. From the one degree-of-freedom model a closed form solution for the plate electromechanical problem is obtained for the first time. Then a strong attention is paid to pull-in phenomena in nonlinear dynamics. Dynamic behavior of the structure is explored, leading to the identification of a pull-in loading condition which is dependent on the quality factor Q. The outcomes are discussed and widely compared with those available in the literature for similar microsystems.

Orientation and size-dependent mechanical modulation within individual secondary osteons in cortical bone tissue

Anisotropy is one of the most peculiar aspects of cortical bone mechanics; however, its anisotropic mechanical behaviour should be treated only with strict relationship to the length scale of investigation. In this study, we focus on quantifying the orientation and size dependence of the spatial mechanical modulation in individual secondary osteons of bovine cortical bone using nanoindentation. Tests were performed on the same osteonal structure in the axial (along the long bone axis) and transverse (normal to the long bone axis) directions along arrays going radially out from the Haversian canal at four different maximum depths on three secondary osteons. Results clearly show a periodic pattern of stiffness with spatial distance across the osteon. The effect of length scale on lamellar bone anisotropy and the critical length at which homogenization of the mechanical properties occurs were determined. Further, a laminate-composite-based analytical model was applied to the stiffness trends obtained at the highest spatial resolution to evaluate the elastic constants for a sub-layer of mineralized collagen fibrils within an osteonal lamella on the basis of the spatial arrangement of the fibrils. The hierarchical arrangement of lamellar bone is found to be a major determinant for modulation of mechanical properties and anisotropic mechanical behaviour of the tissue.

A discrete-time nonlinear Wiener model for the relaxation of soft biological tissues

The present paper is devoted to introducing discrete-time models for the relaxation function of soft biological tissues. Discrete-time models are suitable for the analysis of sampled data and for digital simulations of continuous systems. Candidate models are searched for within both linear ARX structures and nonlinear Wiener models, consisting of an ARX element followed in cascade by a polynomial function. Both these discrete-time models correspond to sampling continuous-time exponential function series, thus preserving physical interpretation for the proposed relaxation model. The estimation data set consists of normalized stress relaxation curves drawn from experiments performed on samples of bovine pericardium. The normalized relaxation curves are found to be almost insensitive to both the magnitude of strain and the loading direction, and so a single model for the whole relaxation curves is assumed. In order to identify the parameters of the Wiener model an iterative algorithm is purposely designed. Over the ARX one, the nonlinear Wiener model exhibits higher capability of representing the experimental relaxation curves over the whole observation period. The stability of the solution for the iterative algorithm is assessed, and hence physical interpretation as material properties can be attached to the parameters of the nonlinear model. Suitable features of the Wiener model for computational application are also briefly presented.

Sensitivity analysis and optimal shape design for bone-prosthesis interfaces in a femoral head surface replacement

Abstract A numerical optimization procedure has been applied for the shape optimal design of a femoral head surface replacement. The failure modes of the prosthesis that were considered in the formulation of the objective functions concerned the interface stress magnitude and the bone remodelling activity beneath the implant. In order to find a compromising solution between different requirements demanded by the two objective functions, a two step optimization procedure has been developed. Through step I the minimization of interface stress was achieved, through step 2 the minimization of bone remodelling was achieved with constraints on interface stresses. The results obtained provided an optimal design that generates limited bone remodelling activity with controlled interface stress distribution. The computational procedure was based on the application of the finite element method, linked to a mathematical programming package and a design sensitivity analysis package.

Unilateral contact problems for cables

Abstract A cable (such as in a submarine energy-transmission line) is supposed to be subjected to given transversal loads and a given tension force, in the vicinity of a frictionless rigid ground of known profile. The search for its equilibrium configuration is a contact (unilateral support) problem of a special kind. The problem is studied here with reference to a finite difference discretization of the system, under the small deformation hypothesis. On this basis its formulation becomes a linear complementarity problem or, alternatively, a quadratic, strictly convex program. Extremum properties of the solution are established and interpreted in mechanical terms. A general monotonicity property of the equilibrium configuration under proportional loading is pointed out; precisely, it is proved that, despite the nonlinearity of the system, whatever the load distribution, as the load factor increases, in any point of the cable the possible contact reaction and the vertical distance from the obstacle will never decrease. Among the mathematical programming algorithms presently available, Cryers systematic overrelaxation is found to be efficient for the numerical solution of large-size problems of the type in question. Some numerical experience is presented.

A Predictive Model for the Elastic Properties of a Collagen-Hydroxyapatite Porous Scaffold for Multi-Layer Osteochondral Substitutes

Damaged articular cartilage can be substituted by porous scaffolds exhibiting tailored mechanical properties and with a suited layer-based design. Reliable predictive models are able to provide a structure–property relationship in the design phase is still an open issue which is of prominent relevance. In this paper, a bottom-up homogenization approach is presented having the purpose to determine the elastic properties of each single layer of a osteochondral porous three-layers scaffold: a top cartilage chondral layer and two mineralized layers: an intermediate and a subchondral bone layer. For the cartilage top layer, dry and wet conditions are considered; while, for intermediate and bone layers only dry conditions are considered. The homogenization model is based on the porosity of each layer and on the elastic properties of the constituent materials, i.e., water, hydroxyapatite (HA) and collagen. The elastic moduli predicted for the mineralized layers are compared with available literature results. The model results obtained on the cartilage layers are validated through flat punch micro-indentation tests carried out on wet and dry samples. The results have shown that the elastic modulus of the mineralized layers is of the order of magnitude of few GPa; whereas, the elastic modulus of the cartilage layer which exhibits porosity higher than 90% is as low as 50 kPa and 300 kPa in wet and dry conditions, respectively. The above results show that the knowledge of the mechanical properties of the basic constituents which are universally known and the porosity of the layers are sufficient information to obtain a reliable prediction of the elastic properties of both mineralized layers and of cartilage layers.